Nature’s capacity to store carbon and thereby curb global warming is steadily eroding as farmers around the world seek to feed more people by opening more land at the expense of forests, says a new study led by researchers at the universities of Minnesota and Wisconsin.

The tradeoff between agricultural production and maintaining nature’s carbon reservoirs — native trees, plants and their carbon-rich detritus in the soil — is more pronounced in tropical regions where ever more of the natural ecosystems succumb to the plow, says the study published online in the Proceedings of the National Academy of Sciences.

“This study is important, because it asks how we make tradeoffs between producing more food and sustaining key aspects of the environment, especially our tropical forests," said a statement by Jonathan Foley, director of the University of Minnesota’s Institute on the Environment, and a co-author on the study.

Carbon is one of the planet’s most abundant elements. It is present in all known life forms, and it moves naturally between the biosphere, oceans and atmosphere in a process that allows the element to be continuously recycled.

Humans have accelerated the process by rapidly converting carbon stocks in trees, other plants and the soil to carbon dioxide, the primary greenhouse gas. The problem is most acute in the tropics where forests act as massive carbon sinks because of their rich diversity and abundance of plant life.

This new study documents the problem in a comprehensive and fine-grained analysis of the world’s existing carbon stocks and global crop yields.

“We analyzed the tradeoffs between carbon storage and crop production at a level of detail that has never been possible before,” said Prof. Stephen Carpenter of the University of Wisconsin Madison, another author of the study.

“The main news is that agricultural production by clearing land in the tropics releases a lot of greenhouse gases per unit of food produced,” Carpenter said.

Compared to the world’s temperate regions, the tropics release nearly twice as much carbon to the atmosphere for each unit of land cleared, said Paul C. West, also of the UW Madison and the lead author of the study.

And when a forest is cleared, not only do you lose more carbon, but crop yields are not nearly as high as they are in temperate areas, he said.

“This creates a kind of ‘double whammy’ for a lot of tropical agriculture: we have to clear carbon-rich ecosystems to create tropical croplands, and unfortunately they often have lower yields than temperate systems,” Foley said. “In terms of balancing the needs of food production and slowing carbon dioxide emissions, this is a tough tradeoff.”

In the tropics, for example, for every ton of crop yield, carbon stocks are diminished by as much as 75 tons. Such attrition makes a strong case for intensifying agriculture on already-converted land instead of putting new fields into production.

But meanwhile, pressure to plant more land is growing fastest in the tropics because of burgeoning population, changing diets, food security concerns, and a rising demand for the raw materials of biofuels.